The present invention relates generally to integrated circuit (IC) designs, and more particularly to electrostatic discharge (ESD) protection circuits for ICs.
Electrostatic discharge (ESD) can enter an integrated circuit (IC) through bond pads, which are connections from outside circuitry to the IC. They are usually used for supplying electric power, electric ground, and electric signals. These electrostatic discharges can be created in many different ways. For example, when parts of an external pad leading to an IC are touched by a person, he or she can create static electricity strong enough to destroy circuitry of the IC. In a metal-oxide-semiconductor (MOS) transistor, the gate oxide is most susceptible to damage. A voltage slightly higher than the supply voltage can destroy the gate oxide of the transistor. ESD created by common environmental sources can carry up to tens of thousands of volts when it occurs. Such voltages can damage the circuitry even though the charge and any resulting current are extremely small. To avoid these voltages damaging an IC, it is important to discharge any static electricity at the moment of its occurrence. In order to protect the IC from ESD, protection circuits are necessary.
An ESD protection circuitry needs to allow an IC to operate normally while providing protection for the IC during ESD events. ESD protection circuitry is typically implemented to the IC at its bond pads. The protection circuit can isolate itself from normal operation of the IC by blocking current from flowing therethrough. During operation of the IC, electric power is supplied to a VDD pad and electric ground is supplied to a VSS pad. Many other pads are assigned to carry electronic signals that are supplied from outside or generated from the IC. During an ESD event, the protection circuitry must quickly conduct current so that the electrostatic charge is conducted to VSS ground, and thus dissipated, before damaging the IC.
One of the commonly used schemes for ESD protection circuit is to implement a string of diodes to provide the necessary voltage drop to prevent normal operation voltage opening up the current path for dissipating ESD charge. The diode string is designed to provide the dissipating path for ESD charge when the high voltage of ESD reaches the diodes. However, the diode string tends to have a high turn-on resistance while also susceptible to current leakage and over voltage issues.
Therefore, desirable in the art of ESD protection circuit designs are additional designs that can provide a dissipating path for ESD charge with a low turn-on resistance.